There have been increased demands for low powered radio communications in a variety of home or building monitoring systems within a restricted area, for example, a fire alarm system where a multiplicity of fire detecting terminals communicate with a remote receiver for transmitting fire data detected at individual sites. In such system, because of that the detecting terminals are expected to send the fire data at only few chances and also because of that the detecting terminals are mounted to a location where no external power supply is available, the detecting terminal is preferred to be powered by an incorporated battery, as shown in a fire alarm system proposed by Japanese Patent Publication No. 2006-343983. In the system, each detecting terminal is configured to have a warning device and also a receiver for receiving the fire detection data from any one of the other detecting terminals through radio communication so that every detecting terminal can issue an alarm when any one of the detecting terminals detect the fire occurrence. Such radio communication system is likely to suffer from various environmental noises including a conflicting radio signal utilized in other radio communication system which have to be distinguished from the fire detection data on the side of the receiver. However, the noise can be distinguished only after processing the received data or signal at an expense of corresponding power on the side of the receiving terminal. For instance, the conflicting radio signal can be designated only after receiving at least a unique word contained in the signal, which necessitates undue consumption of the battery before completing to read the unique word. Thus, the system wastes the battery each time such noise is received, and therefore shortens its operation life. In this sense, the prior art system is not sufficient for reducing battery consumption while the system is idling or waiting for receiving valid data.